Assessment of plaque morphology and composition in carotid vessels is clinically important for early detection of vulnerable plaque, monitoring the progression of atherosclerotic plaque, and response to treatment for such plaque. Multi-contrast, 2D black-blood MRI (also referred to as “dark-blood” imaging) has been established as a non-invasive measure for characterizing the composition of carotid plaque. This vascular imaging technique includes suppression of the signal from flowing blood (rendering it “dark” or “black”) rather than enhancing it as is done in bright-blood techniques. Rapidly-flowing or turbulent blood tends to exhibit a low signal because of phase-dispersion-induced signal losses. These effects may be further enhanced by application of flow-spoiling gradients, saturation bands, and/or inversion pulses. The lack of intraluminal signal allows the walls of vessels to be more clearly delineated during MR imaging. Thus, dark-blood techniques are often used in cardiac imaging and for evaluation of diseases of the vessel wall (e.g., to assess atherosclerotic plaque).
Dark-blood imaging techniques are described, e.g., in H. R. Underhill et al., MRI of carotid atherosclerosis: clinical implications and future directions, Nature Reviews Cardiology 2010; 7:165-173, and in T. Saam et al., Carotid plaque composition differs between ethno-racial groups: an MRI pilot study comparing mainland Chinese and American Caucasian patients, Arteriosclerosis, Thrombosis, and Vascular Biology 2005; 25:611-616. Both of these references are incorporated herein by reference in their entireties. The 2D carotid vessel imaging method described in these references is, however, limited by poor slice resolution and long imaging times.
A three-dimensional (3D) imaging technique for rapid assessment of plaque burden with a balanced steady-state free precession (SSFP) imaging readout is described, e.g., in I. Koktzoglou et al., Diffusion-prepared segmented steady-state free precession: Application to 3D black-blood cardiovascular magnetic resonance of the thoracic aorta and carotid artery walls, Journal of Cardiovascular Magnetic Resonance 2007; 9:33-42, which is incorporated herein by reference in its entirety. A 3D assessment of plaque burden using a spoiled gradient-echo sequence is described, e.g., in N. Balu et al., Carotid plaque assessment using fast 3D isotropic resolution black-blood MRI, Magnetic Resonance in Medicine 2011; 65:627-637, which is also incorporated herein by reference in its entirety. Compared to 2D methods, such 3D approaches can provide the benefits of volumetric spatial coverage and higher imaging efficiency. However, obtaining high-resolution volumetric images requires relatively long imaging times, e.g. on the order of 2-3 minutes. As a consequence, such 3D imaging procedures are frequently challenged by complex motion of carotid arteries originated from multiple sources, including arterial pulsation, swallowing, breathing, and involuntary motion of patient. For example, in a recent multi-center trial—described in L. Boussel et al., Atherosclerotic plaque progression in carotid arteries: monitoring with high-spatial-resolution MR imaging—multicenter trial, Radiology 2009; 252:789-796—carotid MRI results from 52 out of 160 patients had to be excluded from analysis, with motion during the imaging procedure accounting for 46% of all rejections.
Radial k-space sampling schemes can oversample data in the central portion of k-space (e.g., close to the origin), and consequently may result in a reduction of undesirable motion-based image artifacts. For example, an isotropic 3D radial sampling approach with k-space data ordered in a “koosh-ball” type of trajectory can provide improved resistance to motion artifacts, as all data lines pass through the origin of k-space. This radial k-space sampling approach is described, e.g., in C. Stehning et al., Fast isotropic volumetric coronary MR angiography using free-breathing 30 radial balanced FFE acquisition, Magnetic Resonance in Medicine 2004; 52:197-203, which is incorporated herein by reference in its entirety.
In the isotropic 3D radial sampling technique described above, all k-space lines are equally weighted and contribute equally to the final image. However, it can be difficult or impossible to maintain the effect of magnetization preparation when using such an ordering scheme. Specifically, for MRI imaging of a carotid vessel, it is difficult to achieve consistent blood signal nulling and good fat saturation (to isolate these background signals from image data of the vessel walls themselves) using an isotropic 3D radial sampling of k-space.
Accordingly, it would be desirable to have a system and method for 3D magnetic resonance imaging of carotid vessels that addresses some of the shortcomings described above, for example, which can provide good nulling of fat and blood signals and also be insensitive to motion effects.